Wheel bearing apparatus that supports a vehicle wheel relative to a suspension apparatus and that incorporates a wheel speed detecting apparatus to detect rotation speed of the vehicle wheel to control anti-lock braking systems (ABS) are known. Such a bearing apparatus generally includes a wheel speed detecting apparatus with a magnetic encoder having magnetic poles alternately arranged along its circumference. Also, it is integrated in a sealing apparatus arranged between inner and outer members to contain rolling elements (balls) therebetween. A wheel speed detecting sensor detects the variation in the magnetic poles of the magnetic encoder according to the rotation of the wheel.
The wheel speed sensor is usually mounted on a knuckle after the wheel bearing apparatus is mounted onto the knuckle to form a suspension apparatus. Wheel bearing apparatus that incorporates a wheel speed detecting apparatus where a wheel speed detecting sensor is incorporated into the wheel bearing in order to reduce the size of the wheel bearing apparatus as well as to eliminate troublesome air gap adjustment between the wheel speed sensor and the magnetic encoder have been proposed.
An example of a wheel bearing apparatus incorporated with a wheel speed detecting apparatus is shown in FIG. 8. The wheel speed detecting apparatus has a magnetic encoder 51 fit onto an inner ring 50. An annular sensor holder 53 is mounted at one end of an outer member 52 opposite to the magnetic encoder 51. A wheel speed sensor 54 is embedded in a holding portion 57 that forms the sensor holder 53. It is arranged opposite to the magnetic encoder 51 via a predetermined axial gap.
In the description below, the outer side of the bearing apparatus, when it is mounted on a vehicle, is referred to the “outer side” (the left side in a drawing), and the inner side of a bearing apparatus, when it is mounted on a vehicle, is referred to as the “inner side” (the right side in a drawing).
The magnetic encoder 51 comprises a rubber magnet formed of elastomer mingled with magnetic powder, such as ferrite. It constructs a rotary encoder having N and S poles alternately arranged along its circumference to detect the wheel speed. The magnetic encoder 51 is integrally bonded, via vulcanized adhesion, to the side of an annular base 55. The base 55 is formed by pressing a steel plate to have an L-shaped cross-section.
The sensor holder 53 has an annular fitting member 56 mounted on one end of the outer member 52. The holding portion 57 is integrally molded with the annular fitting member 56. The annular fitting member 56 is made by pressing stainless steel that has corrosive resistance. A fitting portion 56a is press fit onto the outer circumference of the outer member 52. A flange portion 56b extends radially inward from the fitting portion 56a. A cylindrical portion 56c axially extends from the flange portion 56b. A plurality of apertures 58 is formed in the cylindrical portion 56c along its circumference so as to firmly grip the integrally molded holding portion 57.
The seal 59 is arranged at the inner side of the magnetic encoder 51 via a sensor holder 53. The seal 59 includes an annular sealing plate 60 and a slinger 61. The sealing plate 60 has a metal core 62 formed with an L-shaped cross-section that is fit into the cylindrical portion 56c of the annular fitting member 56. A sealing member 63 is integrally bonded to the metal core 62, via vulcanized adhesion. The sealing member 63 has an integrally molded side lip 63a, grease lip 63b and middle lip 63c. 
The slinger 61 is made by pressing stainless steel that has corrosive resistance. The slinger 61 has a cylindrical portion 61a press fit into the inner ring 50. A standing portion 61b extends radially outward from the cylindrical portion 61a. An annular tongue portion 61c projects axially from the tip of the standing portion 61b. The tongue portion 61c is opposite to the cylindrical portion 56c of the annular fitting member 56, via a slight gap, and forms a labyrinth seal 64 therebetween. The side lip 63a slidably contacts the standing portion 61b of the slinger 61. The grease lip 63b and the middle lip 63c slidably contact the cylindrical portion 61a of the slinger 61.
According to such a structure, since the wheel speed sensor 54 is embedded in the annular sensor holder 53, the seal 59 is arranged at the inner side of the wheel speed sensor 54. Additionally, the labyrinth seal 64 is arranged at the inner side of the seal 59. Thus, it is possible to prevent the entry of foreign matters, such as magnetic powders, into a space between the magnetic encoder 51 and the wheel speed sensor 54 under severe running conditions of a vehicle. Thus, this improves the reliability of the detection of the wheel speed. (Japanese Laid-open Patent Publication No. 300289/2005).
However, the annular fitting member 56 and the holding portion integrally molded with the annular fitting member 56 may be separated when they are exposed to severe conditions, such as muddy water or salt water or high or low temperature, for a long term. This would cause an increase of the air gap between the magnetic encoder 51 and the wheel speed sensor 54. Thus, the desired exact detection of wheel speed cannot be performed.
In addition, it is impossible to directly apply pressing force on the annular fitting member 56 when this fitting member 56 is press fit onto the outer member 52 using a pressing tool having a substantially C-configuration. The pressing force has to be applied on the end face of the holding portion 57 of an elastic material. This sometimes causes insufficient contact between the annular fitting member 56 and the end face of the outer member 52. Thus, it is difficult to always have a constant air gap between the magnetic encoder 51 and the sensor holder 57.